1. Field of the Invention
This invention relates to a plasma treating method and apparatus therefor, and more particularly to a plasma treating method and apparatus therefor which will be suitable for carrying out the treatment by effecting alternately etching and deposition.
2. Description of the Prior Art
With a progress in miniaturization of semiconductor devices, dimensional machining accuracy of circuit patterns and a low damage machining method have become more and more important. Particularly in devices of a submicron range, the device structure has become three-dimensional owing to limitation to a chip area. For this reason, it has become necessary to form a film having a large ratio of a machining depth to a machining width, that is, an aspect ratio, with high dimensional accuracy.
An example of prior art techniques for satisfying such a requirement is disclosed in Japanese Patent Laid-Open No. 50923/1985, for example. This prior art technique uses a mixed gas consisting of an SF.sub.6 gas which contributes to etching, an N.sub.2 gas which contributes to the formation of a protective film of silicon nitride and other gases as an etching gas for etching poly-Si, and changes periodically the composition and concentration of the treating gas during the etching treatment. In this manner, this prior art technique carries out etching at a high speed and with high dimensional accuracy by repeating alternately the etching step and the formation step of the silicon nitride protective film.
Since the prior art technique carries out the etching treatment by changing the gas composition and concentration, however, the condition of plasma changes whenever the gas composition and concentration are changed. When the gas composition and concentration are changed, a previous plasma condition is changed to a new plasma condition. In other words, any remaining ions and radicals must be exhausted rapidly. Since a treating vessel has a certain inner capacity, a certain period of time is necessary before the plasma condition is changed over so that the overall treating time becomes elongated. If this problem is to be somewhat alleviated, an exhaust device becomes greater in size to reduce the exhaust time. In addition, it becomes necessary to control the exhaust quantity during the treatment and the exhaust quantity at the time of change-over so that a controller and control technique become complicated.
On the other hand, a technique which changes the voltage to be impressed upon an electrode is disclosed in Japanese Patent Publication No. 41132/1986, Japanese Patent Laid-open No. 13625/1986, and the like. These prior arts carry out the plasma treatment by changing the voltage to be applied to the electrode, and merely improve plasma characteristics such as an etching rate, a selection ratio, and the like, by controlling the incidence energy of ions in the plasma.